- Economic Geology
Sir: R.C. Morris, in his discussion of our paper, attempts to defend the supergene-metamorphic model of ore genesis that he was largely instrumental in developing between 1980 and 1990 (see references listed in his discussion). We begin this reply by listing those areas where we agree with Morris on the interpretation of Hamersley province geology, following up with the major areas of disagreement with his model as applied to Mount Tom Price and by extension to other similar orebodies in the Hamersley province.
Firstly, we agree with Morris that the widespread occurrence of hematite pebble conglomerates at the base of the Middle Wyloo Group Mount McGrath Formation indicates that surface enrichments of iron, possibly analogous to the Tertiary martite-goethite ores, may have developed in the early Precambrian on both the Brockman and Marra Mamba Iron Formations, but we also think that these conglomerates are poorly understood. Reconnaissance studies have shown that, like the hematite ores, these conglomerates underwent several stages of recrystallization and hematite formation. Highly variable sizes of microplates of hematite between different pebbles in the conglomerates suggest that at least one phase of microplaty hematite formed prior to the surface exposure and erosion that formed the pebbles.
Secondly, we agree with Morris that the major resource of iron-enriched BIF is located at and immediately below the present ground surface in the laterally extensive martite-goethite bodies developed on both the Brockman and Marra Mamba Iron Formations. Large-scale mining of the Marra Mamba ores has already begun and is set to increase substantially in the future as the higher grade but deeper Proterozoic orebodies are depleted.
Thirdly, we agree with Morris that the major Mount Tom Price and Mount Whaleback orebodies did not form at the Precambrian surface, and there is no direct evidence that they were ever exposed in …